Silicon nitride has been attracting much attention for its favorable properties, and the research thereof has recently led to remarkable developments. It has been already applied commercially to the production of cutting tools, mechanical seals, etc., for its resistance to heat and wear, and will find wide use for bearings, turbosupercharger rotors, etc., in the near future. It will play an important role in the production of gas turbine blades, adiabatic engines, heat exchangers for high-temperature gas furnaces and other equipment which operates at very severe conditions.
It is well known that silicon nitride when formed into fibers exhibits more effectively than in block form its inherent properties, such as mechanical strength, resistance to heat, impacts, oxidation and chemicals, electrical insulation, and wettability with metals. The fibers have another advantage of being more amenable to molding. Economic production of the fibers, therefore, will open up new areas into which silicon nitride can make inroads. More specifically, silicon nitride will be used as a heat-resistant material for mesh belts, conveyor belts, curtains and filters, or as a reinforcing agent in various composite material for engine parts, fan blades and aircraft structures when the fibers are processed into woven fabrics, felts, ropes, yarns or chopped strands.
Various processes have been proposed for the production of silicon nitride fibers. Some of the more important processes are listed below:
(1) a process, wherein silicon monoxide (SiO) prepared by reducing silicate at a high temperature is reacted with ammonia and hydrogen at 1425.degree. to 1455.degree. C. to form fibrous silicon nitride, 5 to 30 .mu.m in diameter and about 370 mm long, on a graphite substrate. (Cunningham et al., 15th Nat. SAMPE Symp., 1969),
(2) a process, wherein an organic polycondensate having one or more silazane (SiN) groups is melt-spun to form a fibrous polymer, which is fired in an ammonia atmosphere (Ishikawa et al., Japanese Patent Laid-Open No. 200210/1982), and
(3) a process, wherein organosilazane fibers prepared by melt spinning or dry spinning organosilazanes are fired in an inert gas atmosphere to produce silicon carbide/silicon nitride composite fibers (G. Winter et al., Japanese Patent Laid-Open No. 69717/1974; W. Verbeek et al., Japanese Patent Laid-Open No. 20206/1974; Penn et al., "J. of Applied Polymer Science", Vol. 27, 3751-3761 (1982); Penn et al., I.E.C., "Proc. Des. Dev.", Vol. 23, No. 2, 217-220 (1984); Seyferth et al. U.S. Pat. No. 4,482,669).
Each of the above processes and silicon nitride fibers produced thereby have specific problems. The process (1) is incapable of making silicon nitride into continuous fibers, and is not suited for mass production because of its poor controllability. The major problems associated with the processes (2) and (3) result from higher carbon contents in their starting fibers, with the result that the final products will have higher concentrations of silicon carbide and/or free carbon. This may sometimes cause formation of cracks, voids and pores during the thermal decomposition step, which will degrade some of the properties inherent in silicon nitride, such as electrical insulation, mechanical strength and resistance to thermal shocks. Furthermore, the fibers prepared by the processes (2) and (3) are insufficient in tensile strength and electrical resistivity, which are known in the art to have to be in the order of 41 to 142 kg/mm.sup.2 and 7.times.10.sup.8 .OMEGA..multidot.cm, to be used for aerospace materials.